Recently, manufacturing technologies of flat panel displays (FPDs) and solar cells have been rapidly developed, and markets of large-screen thin TVs and solar cells have been expanded. Along with the expansion of the markets of these products, glass substrates have been progressively enlarged for the purpose of decreasing the manufacturing cost of the products. Today, manufacturing devices for the eighth generation glass substrates having a size of 2200 mm×2400 mm are being developed. Especially, in the field of sputtering devices for forming a metal thin film or a metal oxide thin film on a large glass substrate, a conventional flat sputtering target is now replaced with a cylindrical (also referred to as “rotary” or “rotatable”) sputtering target. A cylindrical sputtering target has advantages over a flat sputtering target that the use efficiency of the target is high, the erosion occurs less, and generation of particles by delamination of deposited substances occurs less.
A cylindrical sputtering target usable for a sputtering device for forming a thin film on a large glass substrate as described above needs to have a length of at least 3000 mm. It costs high and thus is not practical to manufacture, by integral molding, and grind such a long cylindrical sputtering target. Therefore, usually, a plurality of cylindrical sintered compacts each having a length of several ten millimeters or greater and several hundred millimeters or less are coupled together to form a cylindrical sputtering target.
Generally, a sintered compact, encompassing the above-described cylindrical sintered compact, needs to have a high density in order to have an improved mechanical strength and also in order to allow a thin film formed of the sintered compact to have an improved film quality. In order to realize a sintered compact having a high density, Japanese Laid-Open Patent Publication No. 2014-040348, for example, discloses a technology for increasing the density of a compact to be sintered to a maximum possible level. In a sputtering target including a plurality of sintered compacts coupled together as described above, the difference in the density between adjacent sintered compacts (namely, the “inter-individual sintered compact variance” in the density) influences the sputtering characteristics.
As compared with a flat sintered compact, a cylindrical sintered compact is complicated in the contraction behavior when being sintered. When being contracted greatly, the cylindrical sintered compact is easily distorted. Like with the flat sintered compact, in the case where a highly dense cylindrical compact is formed in order to decrease the contraction amount during the sintering, a binder or the like included in the compact is not easily pulled off, which prevents the compact from having a desired shape as a result of the contraction.